This invention relates to automatic gain control (AGC) in systems using feedback. In particular, it is a method and means of achieving AGC with a substantially constant response time over a relatively wide dynamic range. This is of especial importance in radio receivers in which a signal must typically be controlled over a wide dynamic range, and it has applications in other control systems.
The normal operation of radio receivers calls for the development of an output signal that is constant in level despite variations in the level of the carrier of a received signal. A properly designed receiver will leave the listener unaware of most of the fading or other variations that may occur in a signal to which he is tuned, and it will also leave him unaware of differences in the average received signal strengths of the carriers of different stations to which he may tune. To accomplish these objectives, it is customary to apply AGC to a receiver. This is a feedback system that adjusts the gain of one or more elements in a circuit in response to the level of the carrier in the system. In most such systems, gain is essentially an exponential function of the amplitude of the feedback signal. If the controlled amplifier is a bipolar transistor, the gain is essentially an exponential function of the average value of the base-emitter voltage. However, there are disadvantages that can arise from applying ACG to transistors. When AGC is applied to an amplifier, the input and output impedances of the amplifier are normally changed from their values without AGC. The introduction of AGC may also lead to instability in a transistor stage. A further disadvantage is the fact that changing the operating point of a transistor amplifier may increase the distortion produced by intermodulation in the amplifier. All of the effects listed above are disadvantages that must be overcome. One way of overcoming these disadvantages is to connect a controlled variable attenuator in cascade with an amplifier having fixed gain, and applying feedback control to the variable attenuator. However, this creates the corresponding disadvantage of making the response time of the controller a strong function of the input signal level. This is the case with a PIN diode attenuator in which attenuation is a linear function of signal strength. It would be advantageous to keep the response time independent of signal strength.
It is an object of the present invention to provide a system of automatic gain control that has a substantially constant bandwidth.
It is a further object of the present invention to provide a system of automatic gain control that adjusts to a change in input in a time that is essentially independent of the size of the change.
Other objects will become apparent in the course of a detailed description of the invention.